We propose to continue and extend the scope of our investigations of enzymic mechanisms used by microorganisms to degrade aromatic compounds. These reactions constitute an important segment of the earth's carbon cycle and provide an understanding of the catabolic capabilities and limitations of soil microorganisms which is essential for a biochemical approach to many environmental problems. We have established the biochemistry and enzymology of several meta-fission catabolic pathways for various catechols and for gentisic acid; and although their enzymes are usually specific for substrates of natural origin a particular organism may elaborate enzymes for several routes, selection being determined by the substrate specificity of the hydroxylase used to initiate catabolism of the compound used for growth. We seek to determine the extent of utilization of the recently-elucidated homoprotocatechuate pathway; thus, some Gram positive bacteria appear to use this route for degrading tyrosine. Most previous work was done with pseudomonads but our survey of soil organisms emphasizes that other genera, including species of Bacillus and many thermophilic bacteria catabolize aromatic compounds. We shall investigate new metabolic routes catalyzed by these organisms, such as their conversion of benzoate and 4-hydroxybenzoate into gentisate. Our isolation of dioxygenases for gallic acid will be extended to its homologues and their methyl ethers which are natural products arising from lignin degradation. We shall extend our investigations of the enzymes we have purified, selecting a limited number for determinations of protein structure with the object of establishing data for testing theories proposed for the evolution of microbial catabolic pathways. We shall also study changes in enzyme complements and characteristics that accompany selection of spontaneous mutants that are able to grow at the expense of novel aromatic substrates.